In particular, requirements for high temperature resistant materials or for longer life materials has been desired in the automobile industry. In rubber-made materials, antioxidants have been so far added into rubbers to cope with these requirements for high temperature resistance and for longer life materials. However, almost all antioxidants available in these days have been developed for adding mainly into SBR rubber tires or for various plastics materials. These materials are usually used at a temperature of from about 100 to 150.degree. C., so it is sufficient that the antioxidants used in these materials have a temperature resistance of up to about 150.degree. C., and there have been almost no antioxidants that are assumed to be used at a temperature higher than 150.degree. C. Accordingly, when these antioxidants are used at a temperature higher than 150.degree. C., these antioxidants are apt to volatilize from the surface of the materials making it difficult to prolong the life of the materials.
To avoid such a fault and to make an antioxidant less volatile, increasing the molecular weight of antioxidants has been studied. Some of the commercially available antioxidants at present are considerably less volatile. However, in such a less volatile antioxidant, the dispersibility of the antioxidant in a rubber is lowered owing to the increase of its molecular weight. Also, to realize the low volatility of an antioxidant, an attempt of chemically bonding the antioxidant molecule to a polymer and an attempt of holding the antioxidant to a porous substance have been carried out, but there have been many problems in practical use of these methods and the above-described attempts have not yet been practically used at present.
In particular, in the case of an acrylic rubber, an antioxidant is incorporated into the rubber for the purpose of prolonging the life thereof but the antioxidants proposed until now are yet insufficient and more improvement has been desired.
In order to make an antioxidant function for a long period of time, it is necessary that the antioxidant must remain in a rubber for a long period of time and also it is necessary that the antioxidant that traps a radical therein is stable. To hold an antioxidant in a rubber for a long period of time, an attempt of increasing the molecular weight of the antioxidant, an attempt of bonding the antioxidant to a polymer, etc., have been practiced until now as described above, but in the case of using an antioxidant for an acrylic rubber, in particular for an acrylic rubber as a sealing material, such attempts have not sufficiently been able to give a good result and as the case may be, on the contrary, hasten aging.
In general, aging of a rubber is caused by an oxidation reaction and the oxidation is supposed to be more severe at the surface of the rubber, the molecule of the antioxidant is required to have a moderate mobility toward the rubber surface from inner part of the rubber. However, with increasing the molecular weight of an antioxidant and by bonding an antioxidant to a polymer, the mobility of the antioxidant in the rubber becomes small, which is considered to be the reason that the desired effect is not obtained. Also, in such an antioxidant, it is considered that the radical-trapping capacity per parts by weight of the antioxidant is lowered owing to the increase of the molecular weight of the antioxidant.
On the contrary, when an antioxidant having a relatively low molecular weight and a high mobility in the rubber is used, although the antioxidant has a sufficient mobility in the system and a high radical-trapping capacity per weight parts of the antioxidant, the volatile loss of the antioxidant from the surface of the system or the extraction of the antioxidant from the surface of the system into a contact medium, etc., is large, whereby the sufficient effect cannot be obtained.
Furthermore, it has been performed to improve the heat resistance by combination of plural antioxidants, such as the combination of a primary antioxidant (radical-trapping agent such as an amine based or phenol based antioxidant, etc.) and a secondary antioxidant (peroxide decomposing agent such as a sulfur based or phosphorus based antioxidant, etc.), but in this case, the function and the object of each antioxidant are utterly different.
Also, there is a related art that describes combination of a same kind of antioxidants, the use thereof is not limited to one kind. It describes that antioxidants each having the same performance can be used together, but there is no suggestion on a synergistic effect by combination of them.
In a chloroprene rubber and NBR, as the case may be, the synergistic effect is obtained by combination of amine based antioxidants, but the combination of the amine based antioxidants in the above-described case is not effective in the case of acrylic rubbers. The reason is as follows. That is, because the acrylic rubbers are required to be used at a higher temperature (about 150.degree. C.) than the temperature at which the chloroprene rubbers and NBR are used, and also become hard when heat degradation occurs, there is a fact that conventional antioxidants cannot endure the practical use at such a high temperature.